Cycloisomerizations of allenynes to allenenes have beenstudied in the presence of catalytic amounts of [Hg(OTf)2]in acetonitrile. The catalytic system is quite effective forterminal 1,6-allenynes: allenenes were obtained in reasonableto high yields. However, treatment of allenynes with disubstituents at the allenic terminal carbon yielded a triene and/or allenene as a major product(s) depending upon thesubstituents.